It's a sedge, rather than grass. Looks like a Cyperus species but I'm afraid I'm no use to you on which particular one. Could even be C esculentus which makes the yummy horchata which I hope fortified you on your walk.

Cyperus eragrostis, tall flatsedge. Not native but has naturalized widely and is considered invasive in many areas.

What a pity. No horchata from that one.

Floral, I never knew the 'real' horchata had cyperus in it. I'm assuming you maybe tried some on Spain or somewhere? I've been around Mexicans for 30 years and they never used it or mentioned it. Even to google most recipes for it just have rice, no tubers. Maybe that's why I've never much cared for it because it tasted kind of flat. I initially thought that you were using the cyperus seeds in place of rice. lol

Well, now thanks to you, the real horchata it's on my 'things that I need to taste' list, along with natal plums and hmm?

Thank you all, mosr illuminating